Computers and other devices requiring a visual interface often use liquid crystal displays (LCDs) to display data. Recently color LCDs have come into common usage. Two forms of color LCDs exist: active matrix and passive matrix. Both require light rays from a backlight to generate the colors. The backlight generates an image plane of light beneath the LCD, which in turn generates the color display. In both passive matrix and active matrix systems, the color is generated by a tricolor filter. Active matrix LCDs are more popular because of their excellent image quality, high speed, high contrast ratio, and superior color quality.
As is well known in the art, an active matrix color liquid crystal display (AMLCD) transmits only a small percentage of the light generated by the backlight. Typical active matrix LCDs transmit approximately three percent (3%) of the intensity of the backlight. The intensity of the AMLCD is directly related to the intensity of the backlight. Therefore, to have a high intensity transmitted to the user, the backlight must be very bright: roughly thirty (30) to fifty (50) times as bright as the desired viewer intensity.
For indoor use, the 1000 footlamberts (ft-L) generated by most common backlights is sufficient. This results in a 30 ft-L intensity to the user, which is more than sufficient for indoor use. However, to be readable in sunlit environments, such as a passenger automobile, an aircraft cockpit, or an outside environment visited by field technicians, a much brighter backlight is required. The goal intensity for sunlight readability is 200 ft-L as seen by the viewer.
Moreover, for effective sunlight readability, a special front display filter is added to prevent high-intensity sun light reflections from washing out the LCD image. This front display filter reduces the active matrix LCD's light transmission from about 3% to about 2%. This low transmission rate plus the high intensity level needed for sunlight readability requires the use of a very bright backlight.
However, many common high intensity light sources also generate a great deal of heat. This is a problem because LCDs do not tolerate heat well. LCDs have a relatively narrow range of operating temperatures. At temperatures below 0.degree. C. the crystals' movements become sluggish. Above 75.degree. C. the dielectric constant of the crystals within the LCD changes, thereby reducing the performance of the display. At temperatures at and above approximately 85.degree. C., LCDs reach what is called the "clearing point," where the crystals within the liquid crystal display irreversibly break down. At temperatures below the clearing point, any degradation in performance is reversible. That is, if the LCD is cooled, it will perform as it once did. However, once the LCD reaches the clearing point, cooling the LCD will not reverse the damage. The LCD is unusable.
The "coolness" of an LCD's operation is generally expressed as a luminous efficacy, which is defined as the energy in watts needed to produce the luminous intensity reaching the image plane in lumens per watt (lm/w). A high efficacy backlight will have a high lumens per watt value. To be a suitable source of backlighting, any light source must have a high enough luminous efficacy to be cool enough to prevent the LCD from reaching the clearing point. Thus, brute force luminous intensity is not acceptable because typical high-brightness light sources, such as halogen lamps, generate large amounts of heat.
Moreover, a backlight should have a "low profile;" that is, it should be relatively thin. Some light sources that are bright enough and cool enough are not suitable for use as a backlight because of their thickness. An example of such a device is a cathodoluminescent lamp, which generates 10,000 ft-L in a three inch thick package.
In addition, a backlight should have a long lifetime. Also, a backlight should have a high degree of uniformity across the face of the LCD; that is, the image plane created by the backlight must be substantially uniform. These values are calculated as follows. First, the two extremes of the measured intensity values are averaged to yield an average value, which is considered to be 100% relative intensity. Then the measured values are scaled against that average value to yield intensity uniformity values of 100% .+-.some value. A value within the range of 100% .+-.5% within a one square inch area and 100% .+-.15% within the entire display along either or both axes is acceptable. A value outside that range is considered not suitable.
Additionally, a backlight must emit light in the wavelengths required by the tricolor filter of the AMLCD. Active matrix color LCDs require the backlight to provide light at specific frequencies. A backlight without the required frequencies will cause the LCD to display colors of disproportionate intensity and color shift.
In short, a suitable backlight should have a high luminous intensity, have a high luminous efficacy, have a low profile, have a long lifetime, have a high uniformity, and must emit light in the required wavelengths.
The prior art backlights do not offer all of these qualities. Powder electroluminescent (powder EL) backlights have a short lifetime, a low luminous efficacy, and poor color generation. Typical powder EL backlights have a lifetime of only approximately 500 hours. Even newer high-cost powder EL backlights are limited to approximately 1000 hours of use. Moreover, the luminous efficacy of powder EL backlights is totally unacceptable at 0.1 lm/w. In addition, powder EL backlights are limited in color.
Incandescent lamps and light emitting diodes (LEDs) are expensive, very bulky, nonuniform, and limited in color. They also have a high power consumption and generate tremendous amounts of heat.
Fiber optic backlights are very expensive, very bulky, and have a high power consumption. Fiber optic backlights typically comprise a light source coupled to the LCD matrix via a mesh or web of optical fibers. Bends in the web/mesh cause light to leak out at certain predetermined points. Such an arrangement is costly to manufacture and not uniform in intensity.
Thin film electroluminescent (thin film EL) panels are expensive, have a low luminous efficacy of approximately 2 lumens per watt, have a low intensity uniformity, and are limited in color.
Fluorescent tubes are a likely candidate for an improved backlight because standard fluorescent tubes have an intensity of 8000 ft-L and a maximum luminous efficacy of approximately 80 lumens per watt. However, typical fluorescent tube backlights are far from ideal. With a flat reflector beneath the tube, even a serpentine tube, the uniformity at the image plane is well outside the acceptable values listed above (at useful thicknesses). To achieve a suitable uniformity, either the thickness must be increased beyond the useful range, or a very thick diffuser material must be added between the tube and the image plane. The diffuser absorbs a significant amount of the light energy, thereby reducing the backlight intensity and the luminous efficacy.
Some commercially available laptop computers use fluorescent tubes in an edge-light configuration. Light from two fluorescent tubes located at the edge of the LCD panel is reflected toward the LCD image plane by two ramped reflectors with a very low slope. Such a configuration provides sufficient luminous intensity for some indoor, nonsunlight uses, but does not produce nearly enough luminous intensity for sunlight uses.
Serpentine fluorescent lamps are also known in the art. Serpentine fluorescent lamps are very efficient. Fluorescent tubes have three areas while energized and generating light: the negative glow, the Faraday dark space, and the positive column. The negative glow is seen in common fluorescent lamps as a short red area. The positive column is the longer area that runs substantially the length of the tube and is much more efficient at generating ultraviolet light, which the phosphor in the tube uses to generate white light. Serpentine fluorescent lamps have a high luminous efficacy because they maximize the length of the positive column and minimize the length of the negative glow region. Thus, serpentine fluorescent lamps are an excellent starting point.
However, much of the light emitted by a serpentine fluorescent lamp does not reach the image plane of the LCD. Without a reflector, the tube causes a very nonuniform light distribution at the image plane. Adding a flat reflector beneath the tube improves the light distribution, but the uniformity is still outside the acceptable limits, stated above. The major light absorber is the phosphor on the inside surface of the tube.
There have been many attempts to develop a backlight with all of the above desirable properties. For example, one approach is to provide a backlight that uses multiple gas channels to form a plasma sheet to excite the phosphor within the sheet. Such a device generates about 3000 ft-L, but has a very low luminous efficacy (only approximately 15.6 lm/w).
Another approach is to provide a tube comprising a molded glass sheet in a serpentine shape. Such tubes have a brightness of 2500 ft-L, but are so hot that they require a fan to constantly cool them.
Yet another device is a "cathodoluminescent lamp." The light source is a modified cathode ray tube and obtains a brightness of 10,000 ft-L. However, the unit is three (3) inches thick, which is much too thick to be practical, and the luminous efficacy is so low that it requires constant cooling with a fan.
Thus, as can be seen, none of the available backlight technologies provide a high brightness, high efficacy, high uniformity, and low cost solution to backlighting sunlight viewable active matrix LCDs.